A biventricular pacemaker is a type of cardiac pacemaker that can pace both the right and the left ventricle (typically the lateral wall of the left ventricle). By pacing both right and left ventricles, the pacemaker is able to resynchronize a heart whose opposing walls and right and left ventricles do not contract in synchrony. Biventricular pacemakers have at least two leads, one in the right ventricle to stimulate the septum, and the other inserted through the coronary sinus to pace the lateral wall of the left ventricle. There is typically also a lead in the right atrium to facilitate synchrony with atria contraction. The use of a biventricular pacemaker is generally referred to as cardiac resynchronization therapy (CRT).
Programmable biventricular pacemakers enable optimization of the various time delays between pacemaker timing pulses. This optimization procedure requires the physician or nurse to set delays between the various timing pulses. Its general purpose is to coordinate contraction of the various chambers in the heart to improve overall efficiency and function. This particularly holds true for atrioventricular (A-V) pacing delays whereby the time interval between paced or native atrial contraction can be timed with paced ventricular beat for best cardiac efficiency. Although it is generally believed that both ventricles should contract simultaneously for optimum cardiac performance, a V-V pacing delay is often also required to obtain coordinated contraction.
Most commercial pacemakers are externally programmable via a wireless communication system in which a wand is held near the patient's chest in order to facilitate communications with the implanted pacemaker. Wireless programming allows a physician to adjust the pacing mode for the individual patient, thereby generally providing some control over the filling and/or contraction of the heart both at the time of the initial implant and also on a recurring basis. Unfortunately, procedures used today to optimize pacemaker settings tend to be time-consuming and expensive. Thus, it is believed that many programmable pacemakers are implanted and put into use using default factory settings without optimizing the settings for the individual patient.
The most common method of optimization is echocardiographic-guided CRT optimization (echo-guided CRT optimization). While echo-guided CRT optimization may lead to improvement in cardiac function, it is laborious, expensive and generally inconvenient for the patient. In echo-guided CRT optimization, the patient's heart is ultrasonically imaged and measurements taken from the echocardiogram are used by the physician to adjust pacemaker settings. Such a procedure normally takes two to three hours for both the patient and the physician. Echo-guided optimization has been shown to provide incremental improvement in cardiac function and patient functional class. However, in routine clinical practice the procedure may be incorrectly performed due to time constraints or lack of understanding of cardiac hemodynamics by the operator as well the methodological variability of echocardiographic measurements. This can, in turn, worsen patient symptoms. Hence, routine application of echo-guided CRT optimization is limited.
Efforts have been made in the art to assess the patient's cardiac function during pacemaker optimization without the need for subjecting the patient and the physician to an echocardiogram session. For example, it is known in the art to simultaneously sense ECG electrical and heart sound acoustic signals and process and display such data in order to assist CRT optimization without the need for echocardiography. See, for example, pending U.S. Application Publication No. 2006/0155202 A1, published on Jul. 13, 2006 to Arand et al., which is entitled “Hemodynamic Assessment/Adjustment”; Publication No. US2008/0195168 A1, published on Aug. 14, 2008 to Arand et al., which is entitled “Pacemaker-Patient Hemodynamic Assessment/Adjustment System”; and Publication No. US2008/0195164 A1, published on Aug. 14, 2008 to Arand et al., which is entitled “Pacemaker-Patient Hemodynamic Assessment/Adjustment Methodology”.
The systems described in the above published patent applications use disposable microphones mounted to the patient's chest to produce a phonogram which plots detected heart sound over time. This system generally monitors the function of the left ventricle by comparing the onset of the QRS complex in the patient's electrocardiogram to the sonically detected closure of the mitral valve (the valve that lies between the left atrium and the left ventricle) and to the sonically detected closure of the aortic valve (the valve from the left ventricle to the aorta). These patent applications define the time between the onset of the QRS complex in the electrocardiogram and the closure of the mitral valve as the electromechanical activation time, which is also sometimes referred to in the art as electromechanical delay (EMD). These patent applications explain that a shortened electromechanical activation time generally correlates with improved heart function. These patents also define the time interval between the closure of the mitral valve and the closure of the aortic valve as the left ventricular systolic time, and explain that a lengthened left ventricle systolic time generally correlates with improved heart function as well. The system is also capable of measuring some other parameters as well such as intensity abnormal of heart sounds that correlate with worse cardiac function.
Prior work of one of the inventors of the present invention has involved the use of radial artery tonometry and biventricular pacemaker optimization. The results of some of this work are published in Rafique A M and Naqvi T Z, “Novel Method For Biventricular Pacemaker Optimization By A Radial Artery Tonometer. the case report.” Minerva Cardioangiol. June 2007, 55(3): 385-9 and the use of this device in a series of 60 patients in Naqvi T Z, Rafique A M: Echocardiography-guided pacemaker optimization and radial artery tonometry. J Card Failure 14(7):583-589, 2008. In particular, this work involves the analysis of the patient's radial pressure waveform to determine the length of time between the opening and closing of the aortic valve, which is often referred to in the art as ejection duration (ED). Generally speaking, an optimum ejection duration is about 300 milliseconds in a healthy patient. A lowered ED value can indicate systolic failure. In a healthy subject, a heightened ED value may indicate diastolic failure, whereas in a patient with heart failure, increase in ED in particular to changes in pacemaker timings indicates improvement in heart's pumping function and hence its duration as measured by ED.
The assignee of the present application, AtCor Medical, manufactures the SphygmoCor® system which is able to non-invasively collect peripheral blood pressure pulse waveform data as well as ECG data. The commercial SphygmoCor® system includes a hand-held tonometer that is normally held against the patient's wrist by a pen like device or a wrist band, in order to collect pressure waveform data from the patient's radial artery. The signal from the tonometer as well as the signals from the ECG electrodes are transmitted to a digital signal processing module, and data is then transmitted from the module to a PC which is programmed with data acquisition and analysis software. The commercial SphygmoCor® system is able to determine the ejection duration (ED) from peripheral waveform data via algorithms that detect the onset of the systolic pressure waveform (i.e. corresponding to the opening of the aortic valve) and the incisura in the peripheral pulse waveform (i.e. corresponding to the closure of the aortic valve). The current SphygmoCor® system generally uses the ECG signals to analyze pulse wave velocity and heart rate variability.
During CRT optimization, physicians try to coordinate cardiac muscle contraction in order to minimize the isovolumetric contraction time (IVCT) and increase the ED in the left ventricle. The isovolumetric contraction time interval begins when the mitral valve closes, and ends when the blood pressure within the left ventricle is sufficient to open the aortic valve. The time from the onset of electrical cardiac activity (as marked by the onset of Q-wave of the ECG) and the closure of the mitral valve is termed the electrical mechanical delay (EMD). The combination of EMD and IVCT is referred to in the art as the pre-ejection time (PET) interval, and is a particularly useful parameter for CRT optimization. As mentioned, echo-guided CRT optimization is time-consuming and rather expensive. Also, non-invasive heart sound acoustic monitors are not well suited to detect the opening of the aortic valve and are therefore not well suited to measure either PET or IVCT. In addition, the surrogate of ED called LV systolic time (LVST) as measured by time interval between closure of mitral and aortic valves incorporates IVCT in its measurement. This makes measurement of LVST less reliable than ED as measured by SphygmoCor® system.
The primary purpose of the invention is to provide a convenient, non-invasive means for monitoring the pre-injection time (PET) and ejection duration (ED) of the systolic phase of the heart cycle, thereby enabling medical staff to efficiently optimize programmable settings for an implanted biventricular pacemaker.